Chronic monitoring of heart function can provide valuable diagnostic information for a variety of cardiovascular conditions, including arrhythmias, heart failure, syncope or other autonomic system abnormalities, etc. For example, chronic monitoring of the ECG subcutaneously, using a device such as the Reveal® Insertable Loop Recorder available from Medtronic, Inc., can be useful in diagnosing infrequent symptomatic episodes such as unexplained syncope and for capturing asymptomatic arrhythmias, which may be of interest to the patient's physician. Adding a monitor of hemodynamic function to ECG information would allow a physician to monitor and diagnose a larger spectrum of cardiac conditions that may or may not include arrhythmic events as seen in the subcutaneous ECG. In addition, a hemodynamic signal may allow the physician to continue to track the heart rate when the subcutaneous ECG becomes difficult to interpret due to electrical interference from artifacts such as loss of electrode contact, muscle noise or electromagnetic interference (EMI).
Numerous measurements may be made to gain an indication of hemodynamic function such as blood pressure, blood flow, wall motion, ventricular volume, or other measurements. These types of measurements are commonly made during a clinical examination and may involve both invasive and non-invasive procedures. For example, non-invasive ultrasound measurements may be made to investigate heart chamber volumes, heart wall motion, or blood flow velocities using Doppler ultrasound techniques. Principles and techniques for ultrasound instrumentation and measurements in cardiac applications are known and generally described in: Medical Instrumentation: Application and Design, ed. J. G. Webster (Houghton Mifflin Company, Boston) 1992, pp. 422–436 and Transducers for Biomedical Measurements, ed. R. Cobbold (John Wiley and Sons, New York) 1974, pp. 275–318. While clinical examinations are useful for patient assessment, they are limited in that they reflect the patient's condition only at a particular time on a particular day. Repeated examinations may be performed but the same measurement may not be repeated in exactly the same way because of slight differences in the sensor position and angle of the microphone or ultrasonic probe.
Chronic hemodynamic monitoring may be performed using implantable devices. For example blood pressure can be measured directly by placing a pressure sensor in the cardiovascular system, such as in the right ventricle. A chronically implantable device for monitoring the intracardiac electrogram (EGM) and right ventricular blood pressure has been implanted in chronic heart failure patients and used to continuously monitor their hemodynamic condition. See Magalski A, et al., J. Card. Fail., 2002;8(2):71–3. Apparatus for monitoring syncope including electrodes for recording electrical activity and/or blood pressure data of the patient's heart is generally disclosed in U.S. Pat. No. 6,351,670 issued to Kroll. An ambulatory cardiac diagnostic unit is generally disclosed in PCT Publication No. WO 90/04942, issued to Baker et al., which monitors heart action using intracardiac electrogram and pressure sensed using an intracardiac lead. These systems advantageously provide chronic heart monitoring but involve placement of a lead with pressure sensors in the patient's heart.
An implantable cardiac monitoring and therapy device which chronically monitors a patient's hemodynamic status as indicated by a cardiac contractility parameter which is derived from the result of performing Doppler ultrasound interrogation of cardiac tissue is generally disclosed in U.S. Pat. No. 5,156,154, issued to Valenta, Jr., et al. A Doppler ultrasound transducer affixed to a catheter is implanted within the right heart for assessing the motion of cardiac myofibril tissue within the heart, contributing to the pumping performance of the left ventricle. Other implantable devices for monitoring the hemodynamic status of a patient that include placing an ultrasound probe in the heart are described in U.S. Pat. No. 5,188,106 issued to Nappholz et al., and U.S. Pat. No. 6,421,565 issued to Hemmingson.
Placement of a pressure sensor, ultrasonic probe, accelerometer, microphone, or any other type of hemodynamic sensor, in the heart or anywhere within or along the cardiovascular system is a relatively invasive procedure. Indirect methods of measuring blood pressure, or other indices of hemodynamic function allow a sensor to be placed outside of the cardiovascular system with the measured signal being correlated to hemodynamic status. An implantable pulse generator having a pressure wave sensor mounted in the pulse generator in relation to the proximal end of a pacing lead for sensing pressure waves transmitted from the distal end of the pacing lead is generally described in U.S. Pat. No. 5,702,427, issued to Ecker et al. The pressure wave sensor is employed by a capture verification system to detect pressure waves associated with heart contraction. A pressure sensor is not positioned within the heart itself, however, this system still requires a lead to be placed in the heart to transmit pressure waves.
The use of extravascular sensors for use in determining hemodynamic status in an implantable device has also been proposed, which avoid the invasiveness of placing a hemodynamic sensor in the cardiovascular system. An implantable monitor for monitoring the hemodynamic status of a patient having extravascular sensors including vascular plethysmography, heart and lung sounds, thoracic impedence and ECG is generally disclosed in U.S. Pat. No. 6,409,675, issued to Turcott. A cardiac stimulating apparatus and method is generally described in U.S. Pat. No. 6,366,811, issued to Carlson, that non-intrusively determines an amount indicative of hemodynamic pulse pressure from an accelerometer signal. The accelerometer transmits a signal to the controller associated with fluid and myocardial accelerations of the patient's heart. The amount indicative of pulse pressure is used to optimize cardiac performance.
Acoustic waves, both within and outside of the human auditory range, can provide significant information relating to heart function. Medical Instrumentation: Application and Design ed. J. G. Webster (Houghton Mifflin Company, Boston) 1992, pp. 378–407. Contraction of the whole heart can produce relatively low frequency sound waves, and blood flow and valve opening and closure produce relatively higher frequency sound waves. These sound waves are advantageously transmitted through the body such that they may be measured at a location relative to the heart but not necessarily in the heart. Recording acoustical data, such as heart sounds, ultrasonic echoes or otherwise, on a chronic basis could provide useful information relating to a patient's hemodynamic function. See Luisada et al., “Assessment of Left Ventricular Function by Nonivasive Methods”, AdvCardiol vol. 32, 1985, pp. 111–141. An apparatus for adjusting the timing of pacing pulses delivered by an implanted cardiac pacer based on acoustic intensity of a particular heart sound received from an acoustic sensor, which may be placed against the sternum or in the pectoral region of the chest but outside the thoracic cavity, is described in U.S. Pat. No. 5,554,177, issued to Kieval et al., incorporated herein by reference in its entirety.
It is desirable, therefore, to provide a minimally invasive, implantable hemodynamic monitor that allows acoustical data related to the hemodynamic status of the patient to be recorded and stored. Such data may be used for diagnostic or therapy evaluation purposes and may be evaluated alone or in conjunction with ECG data to interpret both the electrical and mechanical function of the heart.